1. Field of the Invention
The subject invention is directed to fuel injectors for gas turbines, and more particular, to fuel nozzles for gas turbine applications which include an air assist circuit for enhancing fuel atomization during engine ignition.
2. Background of the Related Art
Gas turbines are employed in a variety of applications including electric power generation, military and commercial aviation, pipeline transmission and marine transportation. A common problem associated with gas turbines is the difficulty associated with initiating fuel ignition during engine startup cycles. Moreover, during startup, the fuel must be presented in a sufficiently atomized condition to initiate and support ignition. However, at engine startup, when the engine is gradually spooling up, the fuel and/or air pressure needed to atomize the fuel is generally unavailable.
A broad range of fuel injection devices and methods has been developed to enhance fuel atomization during engine ignition sequences. One approach has been to employ air assist atomizers which utilize high pressure, high velocity air from an external source to augment the atomization process at the low fuel injection pressures that exist during engine startup. Air assist atomizers have been constructed in such a manner so that the externally supplied high pressure, high velocity air is internally mixed with fuel within the nozzle before issuing from the discharge orifice. However, this internal mixing of the air and fuel creates an undesirable back pressure within the nozzle.
Air assist atomizer have also been constructed in such a manner so that the air assist circuit directs high pressure, high velocity air from an external source toward the fuel film so that it impinges upon an outer surface of the fuel film downstream of the discharge orifice. This avoids the back pressure associated with the internal-mixing method, as there is no internal communication between the air and fuel. It is less efficient however, than the internal-mixing concept, and higher flow rates are needed to achieve the same degree of atomization.
Another approach to enhance fuel atomization during engine ignition has been to employ airblast atomizers which function in substantially the same manner as air assist atomizers, in that both utilize the kinetic energy of a flowing air stream to shatter a fuel sheet into fine droplets. The main difference between the two atomization concepts is the quantity of air employed and its atomizing velocity. With air assist nozzles, where the air is supplied from an external or auxiliary compressor or a high-pressure cylinder, rather than from the engine compressor discharge, it is important to keep the airflow rate to a minimum. Furthermore, since there are virtually no restrictions on air pressure for air assist atomization, the air velocity can be very high. Thus, air assist atomizers are generally characterized by their use of a relatively small quantity of very high velocity air.
In contrast, because the air velocity through an airblast atomizer is limited to a maximum value corresponding to the pressure differential across the combustor liner, a larger amount of air is required to achieve good atomization. Most airblast atomizers in use today are of the prefilming type, wherein fuel is first spread out into a thin continuous sheet and then subjected to the atomizing action of a high velocity air.
It would be beneficial to provide an air assist fuel injection method that is more efficient than previously methods of air assist atomization, and which can be employed in conjunction with prefilming air blast atomizers as well as pressure atomizers.
The subject invention is directed to a new and useful air assist fuel injection method for gas turbine engine applications that is adapted to enhance fuel atomization, particularly during an engine ignition sequence, and which can be employed in conjunction with prefilming airblast atomizers as well as pressure atomizers.
More particularly, the subject invention is directed to a new and useful fuel injector that includes a nozzle body having a discharge portion that defines a discharge orifice. The discharge portion includes a fuel circuit for directing a hollow fuel film toward the discharge orifice from a fuel pump powered by the gas turbine. The discharge portion further includes an air assist circuit for directing high pressure, high velocity air toward the fuel film, upstream from the discharge orifice, from a source external to the gas turbine to impinge on an inner surface of the fuel film issuing from the discharge orifice, so as to atomize the fuel.
It is envisioned that the fuel injector of the subject invention may be employed in conjunction with a land-based engine, whereby the air assist circuit of the discharge portion is supplied by an external compressor. It is also envisioned that the fuel injector of the subject invention may be employed with a propulsion engine, such as an aircraft engine, whereby the air assist circuit of the discharge portion is supplied by an external storage tank. In such an instance, the external storage tank is preferably charged by the gas turbine during high pressure operating cycles.
In accordance with a preferred embodiment of the subject invention, the discharge portion of the fuel injector further includes a first air blast circuit for directing engine compressor discharge air toward the fuel film upstream from the discharge orifice to impinge on an inner surface of the fuel film issuing from the orifice, and a second air blast circuit for directing engine compressor discharge air toward the fuel film downstream from the discharge orifice to impinge on an outer surface of the fuel film issuing from the discharge orifice.
The nozzle body of the fuel injector further includes a fuel inlet for admitting fuel into the fuel circuit from the fuel pump, an air assist inlet for admitting air into the air assist circuit for an external source, a first air inlet for admitting air into the first air blast circuit from the engine compressor discharge, and a second air inlet for admitting air into the second air blast circuit from the engine compressor discharge.
The subject invention is also directed to a new and useful method of fuel atomization in a fuel injector of a gas turbine. The method includes the steps of providing a nozzle having a discharge portion defining a discharge orifice, directing a hollow fuel film toward the discharge orifice from a fuel pump associated with the gas turbine, and directing high pressure, high velocity air toward the fuel film upstream from the discharge orifice from a source external to the gas turbine to impinge on an inside surface of the fuel film issuing from the discharge orifice.
The method further includes the steps of directing engine compressor discharge air toward the fuel film, downstream from the discharge orifice, to impinge on an outside surface of the fuel film issuing from the discharge orifice, and directing engine compressor discharge air toward the fuel film, upstream from discharge orifice, to impinge on an inside surface of the fuel film issuing from the discharge orifice. Preferably, the step of directing air toward the fuel film from a source external to the gas turbine occurs during engine ignition.
The subject invention is also directed to an airblast atomization nozzle for a gas turbine. The airblast atomization nozzle includes an outer air cap having an interior chamber. An air swirler is disposed within the interior chamber of the air cap and it has an axial bore extending therethrough. The air cap and the air swirler define an outer airblast circuit therebetween. A prefilmer is disposed within the axial bore of the air swirler and it has an axial bore extending therethrough. A fuel swirler is disposed within the axial bore of the prefilmer and it has an axial bore extending therethrough. The prefilmer and the fuel swirler define a fuel circuit therebetween. A heat shield is disposed within the axial bore of the fuel swirler and it has an axial bore extending therethrough that defines an inner airblast circuit. The heat shield and the fuel swirler define an air assist circuit therebetween. The airblast atomization nozzle further includes a nozzle body having means for delivering fuel to the fuel circuit from a fuel pump associated with the gas turbine, and means for delivering high pressure, high velocity air to the air assist circuit from a supply source external to the gas turbine.
The subject invention is also directed to a pressure atomization nozzle for a gas turbine. The pressure atomization nozzle includes an outer cone having an axial bore extending therethrough. A fuel swirler is disposed within the axial bore of the cone and it has an axial bore extending therethrough. The cone and the fuel swirler define a fuel circuit therebetween for receiving low pressure fuel from a fuel pump associated with the gas turbine. An air swirler is disposed within the axial bore of the fuel swirler. The air swirler and the fuel swirler define an air assist circuit therebetween for receiving high pressure, high velocity air from a supply source external to the gas turbine.
These and other aspects of the subject invention and the method of using the same will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings described hereinbelow.